Reconstitutable control assembly having removable control rods with detachable split upper end plugs

ABSTRACT

A reconstitutable control assembly has a spider structure which supports control rods by detachable attachment joints. Each attachment joint includes a hollow connecting finger on the spider structure and an elongated detachable split upper end plug on each control rod. The split upper end plug includes a pair of separate upper and lower plug portions. The upper plug portion has upper, middle and lower sections, whereas the lower plug portion has upper, middle and lower segments. The upper section of the upper plug portion is rigidly attached to the connecting finger on the spider structure, whereas the lower segment of the lower plug portion is rigidly connected to the control rod. Also, the lower section of the upper plug portion and the middle segment of the lower plug portion have complementary threads defined thereon for rigidly threadably attaching the upper and lower plug portions to one another. Two embodiments of locking features can be used in conjunction with the upper and lower plug portions for locking them together so as to resist their unthreading from one another.

This application is a continuation-in-part of application Ser. No.07/232970, filed 8/17/88, now abandoned.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is hereby made to the following copending applications dealingwith related subject matter and assigned to the assignee of the presentinvention:

1. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayerset al, assigned U.S. Ser. No. 089,384 and filed Aug. 25, 1987, adivision of U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned.

2. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayerset al, assigned U.S. Ser. No. 089,203 and filed Aug. 25, 1987, adivision of U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned.

3. "Reconstitutable Control Rod Spider Assembly" by John M.Shallenberger et al, assigned U.S. Ser. No. 162,841 and filed Mar. 2,1988.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to nuclear reactors and, moreparticularly, is concerned with a reconstitutable control rod spiderassembly having removable control rods with detachable split upper endplugs.

2. Description of the Prior Art

In a typical nuclear reactor, such as a pressurized water type, thereactor core includes a multiplicity of fuel assemblies. Each fuelassembly is composed of top and bottom nozzles with a plurality ofelongated transversely spaced guide thimbles extending longitudinallybetween and attached at opposite ends to the nozzles. Also, a pluralityof transverse support grids are axially spaced along and attached to theguide thimbles. Further, a plurality of elongated fuel elements or rodstransversely spaced apart from one another and from the guide thimblesare supported by the transverse grids between the top and bottomnozzles. The fuel rods each contain fissile material and are groupedtogether in an array which is organized so as to provide a neutron fluxin the core sufficient to support a high rate of nuclear fission andthus the release of a large amount of energy in the form of heat. Aliquid coolant is pumped upwardly through the core in order to extractsome of the heat generated in the core for the production of usefulwork.

Since the rate of heat generation in the reactor core is proportional tothe nuclear fission rate, and this, in turn, is determined by theneutron flux in the core, control of heat generation at reactorstart-up, during its operation and at shutdown is achieved by varyingthe neutron flux. Generally, this is done by absorbing excess neutronsusing control rods which contain neutron absorbing material. The guidethimbles, in addition to being structural elements of the fuel assembly,also provide channels for insertion of the neutron absorber control rodswithin the reactor core. The level of neutron flux and thus the heatoutput of the core is normally regulated by the movement of the controlrods into and from the guide thimbles.

One common arrangement utilizing control rods in association with a fuelassembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patentshows a control rod spider assembly which includes a plurality ofcontrol rods and a spider structure supporting the control rods at theirupper ends. The spider structure, in turn, is connected to a controldrive mechanism that vertically raises and lowers (referred to as astepping action) the control rods into and out of the hollow guidethimbles of the fuel assembly. The typical construction of the controlrod used in such an arrangement is in the form of an elongated metalliccladding tube having a neutron absorbing material disposed within thetube and with end plugs at opposite ends thereof for sealing theabsorber material within the tube.

The spider structure typically includes a plurality of radiallyextending vanes supported on and circumferentially spaced about acentral hub. The vanes are flat metal plates positioned on edge andbeing connected at their inner ends to the central hub. Cylindricalshaped control rod connecting fingers are mounted to and supported bythe vanes, with some of the vanes having only a single connecting fingerand other vanes having a spaced pair of connecting fingers associatedtherewith.

Typically, the upper end plug of each control rod has a threaded outerend which is receivable into a bore in the lower portion of one fingerof the spider structure and threadable into a tapped hole formed in thefinger at the inner end of the bore. The end plug is then secured orlocked therein by a key or pin inserted into the side of the finger andthe end plug and then welded therein. Generally speaking, theabove-described control rod spider assembly is not reconstitutable, thatis, the assembly cannot readily be taken apart and worn or damagedcomponents replaced. Instead, the whole assembly must be discarded.

In many older nuclear reactor plants, the control rods are approachingthe end of their design life. Furthermore, in a number of newer plantswith modified reactor internals, the control rods are experiencingaccelerated wear. In most cases wear occurs over local areas on thecladding of some, but not all, control rods of a given control rodspider assembly. Wear is believed to be caused by coolant flow-inducedvibration. The severity of the clad wear is dependent upon the type ofnuclear plant and internals design and typically occurs only on alimited number of control rods of a given assembly.

Consequently, there is a growing interest in and need forreconstitutable control rod spider assemblies wherein selectedindividual control rods that are prematurely worn or damaged can beremoved and replaced. A spider assembly which allows removal of acontrol rod from a spider structure by severing or cutting off thethreadably fastened top end of the control rod and the fastener isdisclosed in French patent application No. 86/08381. A spider assemblywhich seems to allow removal of a control rod from a spider by removinga set screw and unthreading the upper end plug of the control rod from asupport rod axially movably mounted to a spider vane finger is disclosedin Nakazato U.S. Pat. No. 4,711,756. Spider assemblies which allowremoval of a control rod from a spider structure by deforming a lockingelement on the top end of the control rod are disclosed in U.S. patentsto Edwards et al U.S. Pat. No. 4,314,885 and Walton U.S. Pat. No.4,381,283. However, none of these disclosures would appear to be theoptimum approach to providing a reconstitutable control rod spiderassembly.

SUMMARY OF THE INVENTION

The present invention provides a reconstitutable control rod spiderassembly designed to satisfy the aforementioned needs. Thereconstitutable control assembly of the present invention incorporates aremovable control rod with a split upper end plug for detachablyattaching the control rod to a vane finger of the spider. Specifically,the split upper end plug is composed of two separate upper and lowerportions.

The upper and lower portions of each upper end plug are attachedtogether by screw threading and are torqued to incorporate jointpreload. Also, a locking feature is defined by the portions in order toprevent their loosening during reactor operation. Locking isaccomplished by swaging/crimping a thin-walled upper segment of thelower portion onto a flats-bearing middle section of the lower portionof the upper end plug. The flats are thus provided on the upper portionto prevent subsequent joint loosening. Removal of a control rod isaccomplished by unscrewing the rod which will rotate the lower portionof the upper end plug relative to the upper portion thereof and overcomethe crimp retainer locking feature.

A replacement control rod can then be reinstalled and crimped for rodretention. The thin-walled upper section to be crimped is virginmaterial on the lower portion of the upper end plug on the replacementcontrol rod. The detachable attachment features incorporated by theseparate upper and lower portions of the upper end plug do not adverselyimpact an existing desired feature such as the flexure joint which isretained by the control rod upper end plug.

As an alternative to the above-described locking feature, a radiallyflexible and expandable thread-defining coil can be employed between thethreaded lower section and middle segment of the respective upper andlower end plug portions to provide frictional resistance to rotationonce attachment of the portions has been completed. The coil makes itpossible to rotate the control rod against the frictional resistanceposed by the coil without the need for overriding or overcoming a crimpretainer-type locking feature. Where the crimp locking feature waspreviously employed, the middle section and upper segment of therespective upper and lower end plug portions are of cylindricalconfiguration and form a slip fit type joint instead.

Accordingly, the present invention is directed to an end plug for use infacilitating replacement of a neutron absorber control rod on a controlassembly spider structure. The end plug comprises a pair of separateupper and lower plug portions with the upper plug portion havingintegral upper, middle and lower sections and the lower plug portionhaving integral upper, middle and lower segments. The upper section ofthe upper plug portion is configured for rigid attachment to the controlassembly spider structure for supporting the control rod. The lowersegment of the lower plug portion is configured for rigid connection tothe control rod. Also, the lower section of the upper plug portion andthe middle segment of the lower plug portion are complementarilyconfigured for rigid threaded attachment to one another. Further, themiddle section of the upper plug portion and the upper segment of thelower lug portion are complementarily configured for interlockingattachment to one another so as to resist unthreaded detachment of theupper and lower plug portions from one another.

Also, the present invention sets forth in a reconstitutable controlassembly including a spider structure and at least one control rod, anattachment joint for detachably fastening the control rod to the spiderstructure. The attachment joint comprises: (a) a hollow connectingfinger on the spider structure; and (b) an elongated detachable splitupper end plug on the control rod having a pair of separate upper andlower plug portions. The upper plug portion has integrally-connectedtandemly-arranged upper, middle and lower sections. The lower plugportion has integrally-connected tandemly-arranged upper, middle andlower segments.

More particularly, the upper section of the upper plug portion isrigidly attached to the connecting finger on the spider structure,whereas the lower segment of the lower plug portion is rigidly connectedto the control rod. Also, the lower section of the upper plug portionand the middle segment of the lower plug portion have complementarilythreaded means defined thereon for rigidly threadably attaching theupper and lower plug portions to one another. Further, the middlesection of the upper plug portion and the upper segment of the lowerplug portion have complementary interlocking means defined thereon forlocking the upper and lower plug portions together so as to resistunthreading of the upper and lower plug portions from one another.

Further, the present invention is directed to a modified end plug foruse in facilitating replacement of a neutron absorber control rod on acontrol assembly spider structure. The modified end plug comprises apair of separate upper and lower plug portions with the upper plugportion having integral upper, middle and lower sections and the lowerplug portion having integral upper, middle and lower segments. Also, theend plug includes means in the form of a radially flexible andexpandable thread-defining coil.

Further, the upper section of the upper plug portion is configured forrigid attachment to a control assembly spider structure for supporting acontrol rod, whereas the lower segment of the lower plug portion isconfigured for rigid connection to the control rod. Still further, thelower section of the upper plug portion and the middle segment of thelower plug portion are threaded complementarily to receive therebetweenthe radially flexible and expandable thread-defining coil for threadedengagement therewith in a relationship in which the thread-defining coilfrictionally resists rotation of the upper and lower plug portionsrelative to one another. Finally, the middle section of the upper plugportion and the upper segment of the lower plug portion are configuredcomplementarily for defining a slip fit joint between the upper andlower plug portions.

These and other features and advantages of the present invention willbecome apparent to those skilled in the art upon a reading of thefollowing detailed description when taken in conjunction with thedrawings wherein there is shown and described an illustrative embodimentof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the following detailed description, reference will bemade to the attached drawings in which:

FIG. 1 is a side elevational view, with parts partially sectioned andbroken away for purposes of clarity, of a prior art fuel assembly havinga prior art control rod spider assembly disposed above the fuelassembly.

FIG. 2 is a top plan view of the control rod spider assembly removedfrom the fuel assembly of FIG. 1.

FIG. 3 is an elevational view, with parts partially sectioned, of thecontrol assembly as seen along line 3--3 of FIG. 2.

FIG. 4 is an enlarged fragmentary view, with parts partially sectionedand broken away, of a spider structure of the control assembly of FIG.3.

FIG. 5 is a cross-sectional fragmentary view of a vane and control rodconnecting finger of the spider

structure taken along line 5--5 of FIG. 4.

FIG. 6 is a cross-sectional view of the connecting finger taken alongline 6--6 of FIG. 4.

FIG. 7 is an enlarged fragmentary view of one control rod of the controlassembly of FIG. 3, illustrating the upper end plug thereof.

FIG. 8 is an enlarged fragmentary view, with parts partially sectionedand broken away, of the control assembly of FIG. 3, illustrating a priorart attachment joint between the control rod connecting finger on onevane of the spider structure of the control assembly and the upper endplug of one of the control rods thereof.

FIG. 9 is a cross-sectional view of the upper end plug inserted in theconnecting finger taken along line 9--9 of FIG. 8, illustrating a pinlocking the end plug and finger together.

FIG. 10 is a view similar to that of FIG. 8, but illustrating adetachable split upper end plug on a control rod being constructed inaccordance with the principles of the present invention.

FIG. 11 is an exploded view, on a slightly smaller scale than in FIG.10, of the detachable split upper end plug by itself.

FIG. 12 is an enlarged bottom plan view of an upper plug portion of thesplit upper end plug as seen along line 12--12 of FIG. 11.

FIG. 13 is an enlarged top plan view of a lower plug portion of thesplit upper end plug as seen along line 13--13 of FIG. 11.

FIG. 14 is an enlarged fragmentary side elevational view of the upperplug portion of the split upper end plug of FIG. 11.

FIG. 15 is an enlarged fragmentary longitudinal axial sectional view ofthe lower plug portion of the split upper end plug of FIG. 11.

FIG. 16 is an enlarged fragmentary side elevational view, partly inlongitudinal section, of the split upper end plug of FIG. 10illustrating the upper and lower plug portions attached together.

FIG. 17 is a view similar to that of FIG. 10, but illustrating amodified form of the detachable split upper end plug which employs adifferent type of locking feature.

FIG. 18 is a view similar to that of FIG. 11, but illustrating themodified split upper end plug.

FIG. 19 is a view similar to that of FIG. 14, but illustrating the upperplug portion of the modified split upper end plug.

FIG. 20 is a view similar to that of FIG. 15, but illustrating the lowerplug portion of the modified split upper end plug.

FIG. 21 is a view similar to that of FIG. 16, but illustrating the upperand lower plug portions of the modified split upper end plug attachedtogether.

FIG. 22 is an enlarged side elevational view of a radially flexible andexpandable thread-defining member incorporated in the modified splitupper end plug of FIG. 21.

FIG. 23 is a fragmentary plan view of the member as 1 seen along line23--23 of FIG. 22.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like references characters designate likeor corresponding parts throughout the several views. Also in thefollowing description, it is to be understood that such terms as"forward", "rearward", "left", "right", "upwardly", "downwardly", andthe like, are words of convenience and are not to be construed aslimiting terms. In General

Referring now to the drawings, and particularly to FIG. 1, there isshown an elevational view of a nuclear reactor fuel assembly,represented in vertically foreshortened form and being generallydesignated by the numeral 10. Being the type used in PWR, the fuelassembly 10 basically includes a lower end structure or bottom nozzle 12for supporting the assembly on the lower core plate (not shown) in thecore region of a reactor (not shown), and a number of longitudinallyextending guide tubes or thimbles 14 which project upwardly from thebottom nozzle 12. The assembly 10 further includes a plurality oftransverse grids 16 axially spaced along the guide thimbles 14 and anorganized array of elongated fuel rods 18 transversely spaced andsupported by the grids 16. Also, the assembly 10 has an instrumentationtube 20 located in the center thereof and an upper end structure or topnozzle 22 removably attached to the upper ends of the guide thimbles 14to form an integral assembly capable of being conveniently handledwithout damaging the assembly parts.

As mentioned above, the fuel rods 18 in the array thereof in theassembly 10 are held in spaced relationship with one another by thegrids 16 spaced along the fuel assembly length. Each fuel rod 18includes nuclear fuel pellets 24 and the opposite ends of the rod areclosed by upper and lower end plugs 26,28 to hermetically seal the rod.Commonly, a plenum spring 30 is disposed between the upper end plug 26and the pellets 24 to maintain the pellets in a tight, stackedrelationship within the rod 18. The fuel pellets 24 composed of fissilematerial are responsible for creating the reactor power of the nuclearreactor. A liquid moderator/coolant such as water, or water containingboron, is pumped upwardly through the fuel assemblies of the core inorder to extract heat generated therein for the production of usefulwork.

Prior Art Control Rod Spider Assemblies

Turning now to FIGS. 2 and 3 as well as FIG. 1, there is shown a typicalembodiment of a conventional control rod spider assembly, generallydesignated 32, designed for use in the conventional fuel assembly 10 ofFIG. 1. In its basic components, the control assembly 32 includes aplurality of control rods 34 and a spider structure 36 which supportsthe control rods at their upper ends. The spider structure 36 holds thecontrol rods 34 in a pattern matched to that of the guide thimbles 14which adapts them to be inserted through the top nozzle 22 and downwardthrough the guide thimbles 14 of the PWR fuel assembly 10. The spiderstructure 36 is connected to a control mechanism (not shown) which isoperable in a known manner to move the control rods 34 so as to regulatecore power.

In a typical construction, each control rod 34 of the control assembly32 is composed of an elongated metallic cladding tube 38 having aneutron absorbing material disposed therein and upper and lower endplugs 40,42 attached at opposite ends of the cladding tube 38 forsealing the absorber material therewithin. The spider structure 36 ofthe control assembly 32 typically includes a plurality of radiallyextending flutes or vanes 44 supported on and circumferentially spacedabout a central hub 46. Cylindrical shaped control rod connectingfingers 48 are mounted to and supported by the vanes 44. Some of thevanes 44 have only a single connecting finger 48 attached thereon,whereas other vanes 44 have a spaced pair of connecting fingers 48associated therewith.

Turning now to FIGS. 4-9, there is illustrated one prior art attachmentjoint, generally indicated by the numeral 50, provided between eachcontrol rod connecting finger 48 on the vane 44 of the control assemblyspider structure 36 and the upper end plug 40 of each control rod 34.Typically, the upper end plug 40 of each control rod 34 has a threadedouter end 52. Each connecting finger 48 is mounted to the vane 44 in abayonet-type of welded connection, and has an axial bore 54 formed in alower portion 56 thereof with a smaller-diameter threaded hole 58 tappedtherein at the inner end of the bore 54. The threaded outer end 52 ofthe upper end plug 40 is threadably received in the tapped hole 58 whenthe plug 40 is received within the axial bore 54. The end plug 40 issecured or locked therein by a key or pin 60 inserted through alignedholes 62,64 in the sides of the finger 48 and end plug 40 and thenwelded thereto. Parenthetically, it should be pointed out that the axialbore 54 terminates at the start of an upper portion 66 of eachconnecting finger 48 where the finger connects with the vane 44.

A major disadvantage of this conventional control assembly 32 is that itis not reconstitutable; that is, the assembly 32 cannot readily be takenapart and have worn or damaged components thereof replaced. Instead, thewhole assembly has to be discarded.

However, as mentioned earlier, control rod spider assemblies havingremovable control rods are known in the prior art. One recent controlrod spider assembly that is reconstitutable is disclosed in theaforecited French patent application No. 86/08381. Similar to theabove-described prior art control assembly, the French control assemblyincludes a spider structure with connecting fingers on vanes and aplurality of control rods with upper end plugs having a threaded outerend. However, the attachment joint employed to secure each control rodto one connecting finger is modified somewhat from that described above.Each connecting finger of the French control assembly has an axial boreextending therethrough from end to end. When the upper end plug of onecontrol rod is inserted through the axial bore, its threaded outer endextends above the top end of the finger. A fastener or nut is threadedonto the outer end of the control rod upper end plug until it contactsthe top end of the finger. Then a tubular locking cup formed on thecontrol rod upper end plug above the threaded outer end and extendingabove the threaded nut is deformed radially outward to lock within agroove in the nut to retain the nut thereon.

However disadvantageously, to remove the control rod from the spiderstructure, the portion of the end plug which includes the threaded outerend with the nut fastened thereon must first be severed or cut off.Thus, the fastening nut must be replaced after removal.

Control Rod Spider Assembly of the Present Invention

Turning now to FIGS. 10-16, there is shown particularly in FIG. 10, infragmentary form, a reconstitutable control rod spider assembly,generally designated by the numeral 68 and constructed in accordancewith the principles of the present invention, which provides an improvedarrangement for removing and replacing a control rod. Thereconstitutable control assembly 68 basically includes a spiderstructure 70, a plurality of control rods 72 (only one fragmentarilyshown), and a plurality of improved attachment joints 74 (only oneshown) for detachable fastening the control rods 72 to the spiderstructure 70. Except for the parts thereof to be described hereinafter,the spider structure 70 and control rods 72 of the control assembly 68are identical to that of the prior art control assembly 32 of FIGS. 2and 3.

Each attachment joint 74 includes an elongated connecting finger 76 onthe spider structure 70 and an elongated detachable split upper end plug78 on each control rod 72. The split upper end plug 78 includes a pairof separate upper and lower plug portions 80,82. The upper plug portion80 has integrally-connected tandemly-arranged upper, middle and lowersections 80A,80B,80C, whereas the lower plug portion 82 hasintegrally-connected tandemly-arranged upper, middle and lower segments82A,82B,82C.

Being substantially identical to the prior art attachment joint 50 ofFIG. 8, the connecting finger 76 of the attachment joint 74 of thepresent invention has an axial bore 84 formed in a lower portion 76A ofthe finger 76 so as to open at a lower end 86 of the finger, and aninternally-threaded hole 88 at an upper end of the bore 84. Further, theupper section 80A of the upper plug portion 80 has external threads 90allowing it to be rigidly threadably attached within the threaded hole88 of the connecting finger 76. Also, the lower segment 82C of the lowerplug portion 82 is of a smaller diameter size than an upper open end 92of an elongated hollow tube 94 of the control rod 72 allowing it to bemated and rigidly connected within the control rod tube upper end 92.However, the remaining middle and lower sections 80B,80C of the upperplug portion 80 and the upper and middle segments 82A,82B of the lowerplug portion 82 are different from the upper end plug 40 of the priorart attachment joint 50 of FIG. 8.

More particularly, in the attachment joint 74 of the present invention,the lower section 80C of the upper plug portion 80 and the middlesegment 82B of the lower plug portion 82 have complementarily threadedmeans 96,98 defined thereon for rigidly threadably attaching the upperand lower plug portions 80,82 of the split upper end plug 78 to oneanother. The complementarily threaded means 96 defined on the lowersection 80C of the upper plug portion 80 is preferably in the form ofexternal male threads 96. The complementarily threaded means 98 on themiddle segment 82B of the lower plug portion 82 preferably includesinternal female threads defined in a recess 100 at the inner end of acentral opening 102 extending through the upper and middle segments82A,82B of the lower plug portion 82. The external threads 96 of theupper plug portion 80 are threadably received within the internalthreads 98 of the lower plug portion 82 when the portions are attachedtogether. Further, lower section 80C of the upper plug portion 80 hastwo different diametric sizes. At the smaller one of the two diametersof the lower section 80C is defined the external threads 96. Bridgingthe two diameter sizes of the lower section 80C is an annular shoulder104 which faces toward the lower plug portion 82 and is located justabove the external threads 96. The central opening 102 of the middlesegment 82B of the lower plug portion 82 is larger in diameter than theinternally threaded recess 100 thereof so as to define therebetween anannular ledge 106 located just above the recess 100. The upper plugportion shoulder 104 is capable of being bottomed out in a preloadedcondition against the lower plug portion ledge 106 when the upper andlower plug portions 80,82 are threadably attached together.

Further, in the attachment joint 74 of the present invention, the middlesection 80B of the upper plug portion 80 and the upper segment 82A ofthe lower plug portion 82 have complementary interlocking means 108,110defined thereon for locking the threadably attached upper and lower plugportions 80,82 together so as to resist unthreading thereof from oneanother during normal reactor operation. The complementary interlockingmeans 108 defined on the middle section 80B of the upper plug portion 80is a plurality of flat surfaces 108 formed on the exterior thereof atlocations circumferentially spaced from one another. There arepreferably four flat surfaces 108 being circumferentially displacedabout ninety degrees from one another. The complementary interlockingmeans 110 defined on the upper segment 82A of the lower plug portion 82is a hollow sleeve 110 fitted over and about the flat surfaces 108 anddeformably crimped thereagainst so as to resist unthreading of the upperand lower plug portions 80,82 from one another. However, the sleeve 110is thin-walled so as to reformable back to a generally cylindricalconfiguration upon application of a predetermined torque to the controlrod 72 to rotate the same and unthread the lower and upper plug portions80,82 of the split upper end plug 78 from one another. As can be seem inFIGS. 10, 15 and 16, the upper segment 82A of the lower plug portion 82defining the sleeve 110 is much thinner in wall thickness than themiddle segment 82B thereof.

To summarize, the reconstitutable control assembly 68 of the presentinvention incorporates a plurality of removable control rods 72 eachwith a split upper end plug 78 for detachably attaching the control rod72 to a vane finger 76 of the spider structure 36. Specifically, thesplit upper end plug 78 is composed of two separate upper and lower plugportions 80,82. The upper and lower plug portions 80,82 of each upperend plug 78 are attached together by screw threading and are torqued toincorporate joint preload. Also, the interlocking flat surfaces 108 andcrimped sleeve 110 prevent loosening of the plug portions 80,82 duringreactor operation. Locking is accomplished by swaging/crimping thethin-walled sleeve 110 of the lower plug portion upper segment B2A ontoflat surfaces 108 of the upper plug portion middle section 80B. The flatsurfaces 108 are thus provided on the upper plug portion to preventsubsequent joint loosening.

Removal of a control rod 72 is accomplished by unscrewing the rod whichwill rotate the lower plug portion 82 of the upper end plug 78 beingfixedly attached to the control rod tube 94 relative to the upper plugportion 80 thereof being fixedly attached to the spider vane connectingfinger 79 and overcome the crimp retainer locking feature provided bythe deformed sleeve 110. A replacement control rod can then bereinstalled and crimped for rod retention. The replacement thin-walledsleeve to be crimped is virgin material on the lower plug portion of theupper end plug on the replacement control rod. The detachable attachmentfeatures incorporated by the separate upper and lower plug portions80,82 of the split upper end plug 78 do not adversely impact anyexisting desirable features, such as the flexure joint 112 on the upperplug portion 80 and the intended clearance gap 114 between the flexurejoint 112 and the lower end 86 of the finger 76; these prior artfeatures are retained in the split upper end plug 78 of the presentinvention.

Turning now to FIGS. 17-21, a modified split upper end plug 116 isillustrated having upper and lower plug portions 118,120 and employing adifferent type of locking feature as an alternative to theabove-described crimp retainer-type locking feature. In all respects butthe following ones the modified split upper end plug 116 is identical tothe earlier-described split upper end plug 78.

First, in place of the thin-walled crimpable hollow sleeve 110 of theupper section 82A of the upper end plug lower portion 82 and theflats-bearing middle segment 80B of the upper end plug upper portion 80,a thicker-walled cylindrical upper segment 120A is provided on the lowerplug portion 120 and a cylindrical nonflat-bearing middle section 118Bis provided on the upper plug portion 118. Thus, where the crimp lockingfeature was previously employed, the corresponding middle section 118Band upper segment 120A of the respective upper end plug portions 118,120are of cylindrical configuration and merely form a slip fit type jointinstead.

Second, the threaded lower section 118C of the upper plug portion 118and threaded middle segment 120B of the lower plug portion 120 of themodified split upper end plug 116 are greater in length than thecorresponding threaded lower section 80C and middle segment 82B of therespective upper and lower portions 80,82 of the split upper end plug78. Finally, the modified locking feature is provided by a member 122 inthe form of a radially flexible and expandable thread-defining coildeployed between the threaded lower section and middle segment 118C,120Bof the respective upper and lower end plug portions 118,120. Moreparticularly, the coil 122 threadably fits between and at its respectiveinterior and exterior peripheries makes threaded frictional engagementwith external male threads and internal female threads 124,126 on therespective upper plug portion lower section 118C and lower plug portionmiddle segment 120B.

The coil 122 thus provides frictional resistance to relative rotation ofthe upper and lower plug portions 118,120 once attachment of the plugportions has been completed. However, the coil makes it possible torotate the control rod against the frictional resistance posed by thecoil without the need for overriding or overcoming a crimp retainer-typelocking feature.

As best seen in FIGS. 22 and 23, the radially flexible and expandablethread-defining coil 122 is a screw thread coil insert which per se is acommercial product marketed under the tradename HeliCoil. The lockingaction of the coil is achieved by one or more of the windings 128 of thecoil 122 having a series of straight segments or "chords" 128A. When thethreaded lower section 118C of the upper plug portion 118 enters the"grip" coil 122, these chordal segments 128A flex outward, creatingpressure on the threaded section and providing frictional resistance torotation of the upper plug portion 118 relative to the lower plugportion 120.

It is thought that the present invention and many of its attendantadvantages will be understood from the foregoing description and it willbe apparent that various changes may be made in the form, constructionand arrangement thereof without departing from the spirit and scope ofthe invention or sacrificing all of its material advantages, the formhereinbefore described being merely a preferred or exemplary embodimentthereof.

We claim:
 1. In a reconstitutable control assembly for use with anuclear fuel assembly, said control assembly including a spiderstructure and at least one control rod, an attachment joint fordetachably fastening said control rod to said spider structure, saidattachment joint comprising:(a) a hollow connecting finger on saidspider structure; and (b) an elongated detachable split upper end plugon said control rod having a pair of separate upper and lower plugportions, said upper plug portion having integrally-connectedtandemly-arranged upper, middle and lower sections, said lower plugportion having integrally-connected tandemly-arranged upper, middle anlower segments; (c) said upper section of said upper plug portion beingrigidly attached to said connecting finger on said spider structure,said lower segment of said lower plug portion being rigidly connected tosaid control rod; (d) said lower section of said upper plug portion andsaid middle segment of said lower plug portion having complementarilythreaded means defined thereon for rigidly threadably attaching saidupper and lower plug portions to one another; (e) said middle section ofsaid upper plug portion and said upper segment of said lower plugportion having complementary interlocking means defined thereon forlocking said upper and lower plug portions together so as to resistunthreading of said upper and lower plug portions from one another. 2.The attachment joint as recited in claim 1, wherein:said connectingfinger has an axial bore formed in a lower portion thereof so as to openat a lower end of said finger and a threaded hole at an inner end ofsaid bore; and said upper section of said upper plug portion isexternally-threaded and threadably attached within said threaded hole ofsaid connecting finger axial bore.
 3. The attachment joint as recited inclaim 1, wherein:said control rod has an elongated hollow tube with anupper open end; and said lower segment of said lower plug portion ismated with and rigidly connected to said upper end of said control rodtube.
 4. The attachment joint as recited in claim 1, wherein saidcomplementarily threaded means defined on said lower section of saidupper plug portion and said middle segment of said lower plug portionare respective external male threads and internal female threads.
 5. Theattachment joint as recited in claim 1, wherein said complementarilythreaded means defined respectively on said lower section of said upperplug portion is external threads and on said middle segment of saidlower plug portion is a central opening with a recess at its inner endhaving internal threads which threadably receive said external threadson said lower section of said upper plug portion.
 6. The attachmentjoint as recited in claim 5, wherein said lower section of said upperplug portion has an annular shoulder defined thereon adjacent to saidexternal threads and said middle segment of said lower plug portion hasan annular ledge defined thereon adjacent to said recess, said shoulderbeing bottomed out in a preloaded relation against said ledge when saidupper and lower plug portions are threadably attached together.
 7. Theattachment joint as recited in claim 5, wherein said complementaryinterlocking means defined respectively on said middle section of saidupper plug portion is a plurality of flat surfaces formed on theexterior thereof at circumferentially spaced locations and on said uppersegment of said lower plug portion is a hollow sleeve fitted over saidflat surfaces and deformably crimped thereagainst, said sleeve beingreformable upon application of a predetermined torque to the control rodto rotate the same and unthread said lower and upper plug portions fromone another.
 8. The attachment joint as recited in claim wherein saidupper segment of said lower plug portion is smaller in thickness thansaid middle segment thereof.
 9. The attachment joint as recited in claim1, wherein said complementary interlocking means defined respectively onsaid middle section of said upper plug portion is a plurality of flatsurfaces formed on the exterior thereof at circumferentially spacedlocations and on said upper segment of said lower plug portion is ahollow sleeve fitted over said flat surfaces and deformably crimpedthereagainst, said sleeve being reformable upon application of apredetermined torque to the control rod to rotate the same and unthreadsaid lower and upper plug portions from one another.
 10. The attachmentjoint as recited in claim 9, wherein said flat surfaces arecircumferentially displaced about ninety degrees from one another.